QCM OPERATION IN LIQUIDS - AN EXPLANATION OF MEASURED VARIATIONS IN FREQUENCY AND Q-FACTOR WITH LIQUID CONDUCTIVITY

Recently, several reports have shown that when one side of a quartz cr ystal microbalance (QCM) is exposed to a liquid, the parallel (but not the series) resonant frequency is influenced by the conductivity and dielectric constant of the liquid, The effect is still controversial a nd constitutes a serious complication in many applications of the QCM in liquid environments, One suggestion has been that acoustically indu ced surface charges couple to charged species in the conducting liquid , To explore this effect, we have measured the parallel and the series mode resonance frequencies, and the corresponding Q factors, for a QC M with one side facing a liquid, These four quantities have all been m easured versus liquid conductivity, using a recently developed experim ental setup, It allows the simultaneous measurement of the resonant fr equency and the Q factor of aa oscillating quartz crystal, intermitten tly disconnected from the driving circuit, Based on these results, a s imple model together with an equivalent circuit for a quartz crystal e xposed to a liquid is presented, The analysis shows that-it is not nec essary to infer the existence of surface charges (or other microscopic phenomena such as electrical double layers) to account for the influe nce of the liquids electrical properties on the resonant frequency, Ou r results show that the contacting conductive liquid, in effect, enlar ges the electrode area on the liquid side and thereby changes the para llel resonant frequency. By proper design of the QCM measurement, pert urbing effects due to the liquid's electrical properties can be circum vinted.